The present invention is a photosensitive resin composition, more particularly, a heat-resistant photosensitive resin composition having high sensitivity, excellent adhesiveness and heat resistance as a solder resist-plating resist capable of withstanding the temperature of soldering as well as conditions of plating and suitable for use in the preparation of printed circuit boards.
A solder resist or a plating resist is a resist material used in the manufacture of printed circuit boards to protect the printed circuit. Since solder resists are used to provide a permanent resist, they must have excellent properties of heat resistance, adhesiveness, resistance against chemicals, electric insulation and so on. Solder resist materials conventionally used in the prior art include heat-curable melamine-based and epoxy-based materials, and non-solvent type ultraviolet-curable materials which need no heat treatment for curing. Ultraviolet-curable solder resists are used mainly in the manufacture of printed circuit boards in household electric appliances due to their relatively low reliability in adhesiveness. Printed circuit boards in industrial instruments are manufactured mainly by use of heat-curable epoxy-based solder resists. The requirements for plating resists are similar to those for the above described solder resists. Thermosetting epoxy-based resins are used as the plating resist.
These solder resists and plating resists are supplied usually in an ink-like form suitable for screen painting, and a printed circuit board having circuit lines is printed therewith through a screen patterned beforehand. A problem inherent in the screen printing process is the more or less unavoidable displacement of the printed pattern. Thus screen printing process using such a heat-curable solder resist to the manufacture of printed circuit boards for industrial use is increasingly difficult as the density of the printed circuit has increased rapidly in recent years.
Those in the art have eagerly sought in recent years to develop an ultraviolet-curable photosensitive solder resist and plating resist having a high resolving power. Among the various types of the ultraviolet-curable solder resists hitherto proposed, are dry film-type solder resists which are currently available on the market. Though displaying excellent heat resistance, dry film-type solder resists have a problem. When one is laminated to the surface of an already patterned printed circuit board raised and recessed from area to area, every nook and corner cannot always be filled with the resist material leaving air bubbles due to the relatively low adhesiveness of the dry film resist. Such air bubbles located below the dry film resist may expand with heat in the subsequent soldering process to cause so-called blistering. Similar problems attend the use of a dry-film type resist material such as the plating resist.
Further, U.S. Pat. No. 4,390,615 teaches a method of using a photoresist comprising:
(a) an ethylenically unsaturated polymerizable reaction product of a polyepoxide and an ethylenically unsaturated carboxylic acid, which product is solid or semisolid in nature;
(b) an inert inorganic filler, the filler and the reaction product (a) being present in a weight ratio of from 20 to 65 parts of the filler to from 80 to 35 parts of the reaction product;
(c) a photopolymerization initiator for the polymerizable reaction product; and
(d) a volatile organic solvent for the polymerizable reaction product.
The photoresist compositions of this type, however, possess insufficient resistance against organic solvents and cannot satisfy the requirements for the solder resist because of their relatively low heat resistance.